Actuator

Abstract: An actuator has a linearly elongate frame (12), side (14a,14b), end (16,17), and top covers detachably and selectively mounted on side surfaces of the frame (18), a table mechanism (30) displaceable in the axial direction of the frame in response to energization of an electric motor, and a ball screw (24) for transmitting drive forces from the electric motor to the table mechanism. The operator can select various actuator configurations depending on the environment in which the actuator is to be installed and the application in which the actuator is to be used.

Description: BACKGROUND OF THE INVENTION The present invention relates to an actuator according to the preamble of claim 1. Description of the Related Art: There has heretofore been known an actuator having, as a drive source, an electric motor equipped with a sensor such as an encoder, a tachometer generator, or the like for making it possible to control a rotational speed, a drive torque, a stop position, or the like. The rotational motion from the electric motor is converted into linear motion by a drive force transmission shaft such as a ball screw, a trapezoidal screw, or the like, and the linear motion is transmitted to a table mechanism, which is displaced to convey a workpiece. An electric motor which is incorporated in such an actuator is illustrated in longitudinal cross section in Fig. 1 of the accompanying drawings. As shown in Fig. 1, the electric motor, generally denoted at 1, has a coupling 4 interconnecting an end of a ball screw 2 and an end of a motor shaft 3 coaxially with each other. The coupling 4 serves to absorb a misalignment between the axes of the motor shaft 3 and the ball screw 2, and prevent vibrations which are produced when rotary motion is converted into linear motion and also vibrations which are produced due to rotary motion containing a flexural component perpendicular to the axis as the ball screw 2 becomes longer, from being transmitted to the motor shaft 3. One end of the ball screw 2 is rotatably supported by a first bearing 5 comprising balls 5a which are obliquely interposed between inner and outer races thereof. Since the balls 5a bear loads at a certain angle with respect to the axis of the ball screw 2 as indicated by the broken lines in FIG. 1, the balls 5a are capable of absorbing a load in a direction substantially perpendicular to the axis of the ball screw 2 and also a load in a direction substantially parallel to the axis of the ball screw 2. The motor shaft 3 which is housed in a motor housing 6 has an end rotatably supported by a second bearing 7 and a spring washer 8, and an opposite end rotatably supported by a third bearing 9, which is of a double bearing configuration. The first bearing 5 supports the ball screw 2 in both axial and radially inward directions. The second bearing 7 supports the motor shaft 3 in a radially inward direction for thereby absorbing vibrations and inertial forces that are generated in a radially outward direction by the rotational forces of the motor shaft 3. The third bearing 9 supports the motor shaft 3 in both axial and radially inward directions. Therefore, in the case where a photosensor P is mounted in the electric motor 1 for detecting the number of revolutions, the rotational speed, or the like of the electric motor 1, it is possible to position an encoder disk D fixedly mounted on the motor shaft 3 accurately within a clearance A in the photosensor A. The electric motor 1 which tends to experience a relatively high temperature during operation suffers the problem of different thermal expansions due to different materials and shapes of the parts used. Typically, the motor housing 6 is made of an aluminum-base material for heat radiation, and the motor shaft 3 is made of an iron-base material. The difference between different thermal expansions of the materials of the motor housing 6 and the motor shaft 3 causes the motor housing 6 to be displaced axially, possibly concentrating stresses on the second bearing 7 which supports the motor shaft 3. Consequently, it is necessary to absorb the difference between these different thermal expansions in some way. In the conventional electric motor 1, the spring washer 8 is interposed between balls 7a of the second bearing 7 and an inner wall surface of a bracket 6a of the motor housing 6. The difference between the different thermal expansions can be absorbed when the spring washer 8 is elastically deformed, pressing the balls 7a in a direction substantially parallel to the axis of the motor shaft 3. Actuators for making rotary and linear motion, such as an electric actuator represented by an electric motor and a fluid pressure actuator represented by a fluid cylinder, are controlled by a motor driver and a solenoid-operated valve. These actuators, which include actuators for use in robots, are usually disposed independently of, not integrally with, a controller. If the electric motor 1 is incorporated as an actuator in an apparatus (not shown), thus providing a drive source, it is necessary to reduce the size and weight of the actuator as much as possible in order to increase the versatility of the actuator. The coupling 4 which interconnects the motor shaft 3 and the ball screw 2 may bring about resonance in the motor shaft 3 and the ball screw 2 when rotated. When the electric motor 1, which requires high dynamic characteristics, as with a servomotor, resonates, the positional control accuracy thereof is lowered, and its dynamic characteristics are impaired. If the coupling 4, which serves to prevent vibrations from being applied to the motor shaft 3, were dispensed with and the motor shaft 3 and the ball screw 2 were integrally coupled directly to each other in order to alleviate the above drawbacks, then unwanted vibrations would be transmitted to the motor shaft 3. As a result, it would be difficult to convey a workpiece continuously stably. The document WO 93/05321 A1 (corresponding to EP 0 601 185 A1) discloses an actuator according to the preamble of claim 1. In particular, in this actuator a motor, a ball screw shaft and so forth are accommodated in an outer frame, and direct-drive guides are provided on both sides of the groove-like opening formed in this frame. The ball screw shaft is provided at the central portion of the frame, and a table slides along the direct-drive guides by virtue of rotational action of the ball screw shaft. The electric motor and an encoder unit are mounted on an end of the ball screw shaft and housed in a casing. In that casing the ball screw shaft is supported by bearings mounted at opposite ends thereof. SUMMARY OF THE INVENTION It is an object of the present invention to provide an actuator which allows maintaining the clearance between the sensors and the encoder disk accurately under axial load of the shaft. According to the present invention, there is provided an actuator comprising the features of claim 1. Advantageous embodiments of the invention are evident from the dependent claims. The actuators may have respective frames, the frames housing power supply buses and signal buses which interconnect the components. The signal buses may be connected to each other in a wireless configuration. The actuator system may further comprise a network communication system interconnecting the components. The operator can freely select and mount covers of different shapes on the frame depending on the environment in which the actuator is to be installed and the application in which the actuator is to be used. Since the covers are detachable from the frame, the actuator can be modified into a different configuration by removing or adding some of the covers when the actuator is to be installed in a different place. Vibrations produced by the drive force transmitting shaft when the motor is energized are absorbed by the first bearing, and hence prevented from being transmitted to the motor. When a displacement is generated due to the difference between thermal expansions of the casing and the rotatable shaft at the time the motor is heated, the resilient member is elastically displaced to absorb the displacement, thereby preventing stresses from being applied to the first bearing in the axial direction of the drive shaft. The second bearing is capable of absorbing a load in the axial direction and a load in a direction perpendicular to the axial direction. In the actuator system, each of the components of the actuator system is associated with a controller for recognizing itself. Therefore, the actuators can easily be changed. The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example. BRIEF DESCRIPTION OF THE DRAWINGS Fig: 1 is a cross-sectional view of a conventional electric motor incorporated in an actuator; Fig. 2 is a perspective view of an actuator according to an embodiment of the present invention; Fig. 3 is a perspective view, partly omitted from illustration, of the actuator shown in Fig. 2; Fig. 4 is a cross-sectional view of the actuator shown in Fig. 2; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An actuator according to an embodiment of the present invention is shown in Figs. 2 through 4. The actuator, generally denoted at 330, comprises a frame 336 having a plurality of substantially parallel long grooves 334 of substantially T-shaped cross section defined in side surfaces thereof except for a side surface with an opening 332 defined therein, a linear guide 338 disposed on the bottom of the frame 336, a table mechanism 342 linearly displaceable along the linear guide 338 upon rotation of a ball screw shaft 340, and an electric motor 344 coupled to an end of the frame 336. As shown in Fig. 3, the table mechanism 342 comprises a bearing 346 slidable along the linear guide 338, and a table 349 fixed to the bearing 346 and linearly displaceable in response to rotation of the ball screw shaft 340 which is threaded in a ball screw bushing 348 mounted in the table 349. As shown in Fig. 4, the electric motor 344 comprises a motor unit 352 and an encoder unit 354 which are housed in a casing 350. The casing 350 includes a frame 358 having a bracket 356 and an end cover 360 attached to the frame 358. The casing 350 houses a drive shaft 362 coaxially integral with the ball screw shaft 340, the drive shaft 362 serving as a motor shaft. The drive shaft 362 is rotatably supported by a first bearing 364 retained by the bracket 356 and a second bearing 368 retained by an encoder frame 366 (described later on). The first bearing 364, which comprises a deep-groove ball bearing, is axially supported by a spring washer 370 that is located laterally of the first bearing 364. The first bearing 364 and the spring washer 370 are of substantially the same structure as those of the electric motor 2 in the conventional actuator 1 shown in FIG. 1, but differ therefrom as follows: The first bearing 364 is positioned closely to a region in which the drive shaft 362 is exposed out of the casing 350, and holds the drive shaft 362 radially inwardly to maintain a clearance highly accurately between a permanent magnet 372 and a ring-shaped stator coil 374 of the motor unit 352 and also to prevent vibrations caused by flexural movement of the ball screw shaft 340 from being transmitted to the motor unit 352. Displacements generated due to the difference between thermal expansions of the materials of the frame 358 and the drive shaft 362 at the time the electric motor 344 is heated are absorbed by the spring washer 370 when it is axially displaced owing to its own elastic deformation. As a result, the first bearing 364 is prevented from suffering axial stresses therein. The motor unit 352 also has a ring-shaped motor rotor 376 and a stator core 378. The ring-shaped motor rotor 376 and the permanent magnet 372 are successively radially fitted over the drive shaft 362, and the ring-shaped stator coil 374 and the stator core 378 are successively radially fixed to an inner wall surface of the frame 358 which extends around the permanent magnet 372. The drive shaft 362 and the motor rotor 376 are fixed to each other by set screws (not shown) which are threaded through the motor rotor 376 and held against flat surfaces 379a, 379b formed on the drive shaft 362. Alternatively, the drive shaft 362 and the motor rotor 376 may be fixed to each other by a key slot and a key, a retaining ring, a collar, an adhesive, or the like. Further alternatively, a magnet may be mounted directly on the drive shaft 362, and thereafter magnetized thereby integrally forming the drive shaft 362 and the motor rotor 376 with each other. With these fixing arrangements, it is possible to remove the drive shaft 362 out of the casing 350. The encoder unit 354 includes an encoder frame 366 fixed to an inner wall surface of the end cover 360. The second bearing 368 is supported in the encoder frame 366 by a bearing holder 380. The second bearing 368 comprises a double angular ball bearing having two parallel arrays of balls 369. The two parallel arrays of balls 369 of the second bearing 368 are inclined at respective angles to a line perpendicular to the axis of the second bearing 368, so that they are capable of absorbing a load in the direction of the axis of the second bearing 368 and also a load in a direction perpendicular to the axis of the second bearing 368. The second bearing 368 may be replaced with another combined angular ball bearing, a combined conical roller bearing, a double conical roller bearing, or the like. The encoder unit 354 has a pair of boards 384a, 384b spaced from each other and fastened to the encoder frame 366 by pins 386, and a pair of photosensors 388a, 338b secured to the boards 384a, 384b, respectively, the photosensors 388a, 388b serving as a light-emitting element and a light-detecting element, respectively. A nut 390 is threaded over the distal end of the drive shaft 362, and a doughnut-shaped encoder disk 392 is fixedly mounted on and around the nut 390. The encoder disk 392 has a plurality of slits (not shown) positioned between the photosensors 388a, 388b. Light emitted from the light-emitting element passes through the slits and is then detected by the light-detecting element for detecting the rotational speed of the motor 344 and the position of the table 349. Operation and advantages of the actuator 330 will be described below. A power supply (not shown) electrically connected to the motor 344 through leads is turned on to energize the motor unit 352 for thereby rotating the drive shaft 362 in a predetermined direction. The rotary motion of the drive shaft 362 is transmitted through the ball screw shaft 340 to the ball screw bushing 348, which is linearly moved along the ball screw shaft 340 to displace the table 349 therealong. While the motor 344 is in operation, the first bearing 364 absorbs vibrations of the ball screw shaft 340 and hence prevents vibrations from being transmitted to the drive shaft 362. Accordingly, the motor unit 352 can stably continuously rotate the drive shaft 362 without being damaged. When the motor 344 is heated during its operation, the frame 358 tends to be displaced in the axial direction of the drive shaft 362 because of the different thermal expansions of the materials of the frame 358 and the drive shaft 362. However, such a displacement can be absorbed by the spring washer 370 when it is axially elastically deformed. Consequently, axial stresses are prevented from being imposed on the first bearing 364. Since the second bearing 368 supports the drive shaft 362 in a direction perpendicular to the axis thereof and also in the axial direction thereof, the desired clearance between the photosensors 388a, 388b can be maintained highly accurately, allowing the encoder disk 392 to rotate accurately and reliably between the photosensors 388a, 388b. The encoder unit 354 can be dislodged from the drive shaft 362 when the end cover 360 is detached from the frame 358. In the electric motor 344 integrally incorporated in the actuator 330, the ball shaft screw 340 and the drive shaft 362 are integrally formed with each other without any coupling used therebetween. Therefore, the electric motor 344 can be reduced in size as it requires no space for accommodating such a coupling, and can rotate the drive shaft 352 without producing extra inertial forces. As a result, the actuator 330 can be reduced in size and weight for increased versatility.

Claims: An actuator comprising: an electric motor (344) having a rotatable shaft (362); a drive force transmitting shaft (340) for converting rotary motion from said electric motor (344) into linear motion; and a table mechanism (342) movable by the linear motion from said drive force transmitting shaft (340) to convey a workpiece carried on said table mechanism (342); said rotatable shaft (362) and said drive force transmitting shaft (340) being integrally formed with each other; a casing (350) which houses both the electric motor and an encoder unit (354), said encoder unit (354) comprising a pair of sensors (388a, b) mounted in said casing (350) and further comprising an encoder disk (392) interposed between said sensors (388a, b), said encoder disk (392) being integrally coupled to said rotatable shaft (362); and first and second bearings (364, 368) said rotatable shaft (362) being supported at opposite ends thereof by said first and second bearings (364, 368); characterized in that said second bearing (368) comprises a double angular ball bearing having two parallel arrays of balls (369); and that an encoder frame (366) is housed in said casing (350), said pair of sensors (388a, b) being mounted on said encoder frame (366), wherein said second bearing (368) is supported in said encoder frame (366). An actuator according to claim 1, wherein said first bearing (364) is disposed near said drive unit (352) and said second bearing (368) is disposed near said detector (354), said first and second bearings holding said rotatable shaft (362) axially and radially thereof. An actuator according to claim 1, wherein said first and second bearings (364, 368) maintain a predetermined clearance between said sensors (388a, b) and said encoder disk (392). An actuator according to claim 1, further comprising a resilient member (370) for absorbing a displacement produced due to different thermal expansions of said casing (350) and said rotatable shaft (362). An actuator according to claim 1, wherein said rotatable shaft (362) is removable from said casing (350). An actuator according to claim 1, wherein said casing comprises: a primary casing (358) which houses said electric motor (352); and an end cover (360) detachably coupled to said primary casing (358) by fitting over a portion of said encoder frame (366).

Inventors: NAGAI SHIGEKAZU, SUZUKI MASAHIKO, NAGAI, SHIGEKAZU, SUZUKI, MASAHIKO, Nagai, Shigekazu, SMC K. K. Tsukuba Gijutsu Center, Suzuki, Masahiko, SMC K. K. Tsukuba Gijutsu Center
Applicants: SMC KK, SMC KABUSHIKI KAISHA
Application year: 1994
Main classification: B23Q37
IPC codes: B23Q5, B23Q37, F16H25, B23Q1